1. Field of the Invention
This invention relates to a wavelength conversion-based illumination devices, and in particular, it relates to such illumination devices which use multiple excitation light sources to excite a wavelength conversion material to increase brightness of the illumination device.
2. Description of the Related Art
To achieve high brightness, low cost color illumination devices, technologies that use excitation light and wavelength conversion materials are widely used. In such an illumination device, short wavelength light (e.g. blue or UV light) from excitation light sources such as semiconductor lasers or light emitting diodes (LED) are absorbed by the wavelength conversion materials such as phosphors and converted to different wavelength lights. To increase the brightness (i.e. power density) of the illumination device, techniques are developed to increase the efficiency of light conversion by the wavelength conversion materials and the efficiency of light extraction from the wavelength conversion materials.
In one example, to increase light extraction from the wavelength conversion material, the wavelength conversion may be placed in a reflective cavity, and excitation light sources are located on the surfaces of the cavity. The output orifice of the cavity is smaller than the area of the wavelength conversion material, so that the low brightness light generated y by the wavelength conversion material is concentrated by the reflective cavity to obtain a brighter output light. A shortcoming of this technique is that, because the reflectivity of the cavity surface is less than 100%, multiple reflections within the cavity causes significant loss of light, which lowers the efficiency of the illumination device. In addition, when such a device is used for high power illumination devices, the power density generated by the wavelength conversion material is high, so that heat dissipation may become a problem. It has been shown that the light emission efficiency of phosphors decreases with temperature. For example, for a green phosphor material, the light emission efficiency drops significantly when the temperature reaches a certain level such as 130 degrees C. Thus, this type of design has limited efficiency for high power illumination devices.